The present invention relates to apparatus for conditioning tobacco or similar materials including reconstituted and artificial tobacco. More particularly, the invention relates to improvements in apparatus for reducing the moisture content of natural, reconstituted and/or artificial tobacco to a preselected value while the tobacco is advanced along a predetermined path by a transporting unit, preferably a transporting unit which includes a hollow rotary dryer defining a conditioning zone for successive lengths of a preferably continuous stream of particles of tobacco or the like (hereinafter called tobacco for short). Still more particularly, the invention relates to improvements in apparatus wherein the wall or walls of the aforementioned rotary dryer are preferably heated by a fluid medium, such as steam and/or hot air, in order to enable the wall or walls to transfer heat to tobacco particles in the dryer and to thereby expel moisture from such material.
It is already known to utilize a rotary drum-shaped dryer for conditioning of successive increments of a continuous tobacco stream in order to reduce the moisture content of tobacco to a preselected value which is best suited for further processing of tobacco in a cigarette rod making machine or the like. The means for heating the wall or walls of the rotary dryer can comprise elongated pipes or plates which convey steam or another heated fluid (such as a hot gas or hot oil) in order to heat the wall or walls as well as to directly heat the tobacco particles which come in contact therewith. Such pipes or plates can constitute, or perform the function of, orbiting blades or paddles which agitate the constituents of the tobacco stream during travel through the conditioning zone in order to ensure a more uniform heating and drying action. The pipe or pipes and/or the plate or plates can be said to constitute component parts of the wall or walls, i.e., constituents of the rotary dryer, because they also transmit heat from the fluid heating medium to the particles of tobacco in the conditioning zone. Reference may be had to commonly owned U.S. Pat. No. 3,429,317 granted Feb. 25, 1969 to Hans Koch et al.; this patent describes and shows a rotary drum-shaped dryer which constitutes one element of a tobacco transporting unit and whose cylindrical wall is heated by axially parallel pipes connected to a source of hot steam. In the apparatus which is described and shown in the patent to Koch et al., a detector monitors the initial moisture content of tobacco and the signals which are generated by such detector are utilized to regulate the heat content of a hot air stream which is admitted into the inlet of the conditioning zone, i.e., into the tobacco-receiving end of the rotary drum-shaped dryer. The heat content of steam which is used to heat the cylindrical wall of the dryer, and which furnishes the major part of the heating and drying action, is regulated in dependency on deviations of the monitored final moisture content of dried tobacco from a preselected value. The quantity of hot air which is admitted into the conditioning zone of a modern tobacco dryer should be as low as possible and the temperature in the conditioning zone should be very high. Such mode of drying cannot be achieved with the apparatus of Koch et al. because the patented apparatus requires substantial quantities of hot air in order to immediately compensate for pronounced fluctuations in the initial moisture content of tobacco.
If the heating action of a conventional dryer upon the particles of tobacco in the drying or conditioning zone is to be reduced, for example, because the quantity of moisture which is to be expelled from tobacco per unit of time is reduced (this takes place when the quantity of tobacco particles per unit length of the tobacco stream and/or the initial moisture content of tobacco particles decreases), it is necessary to reduce the pressure of steam which is used to heat the wall or walls of the dryer. This creates problems when the pressure of steam decreases to and/or below a certain value. For example, if the pressure of steam which is used to heat the wall or walls of the rotary dryer drops to or below 1 bar, this eliminates the possibility of maintaining a predictable (unequivocal) relationship between the steam pressure and steam temperature on the one hand and the drying action on the other hand. Therefore, automatic dryers are normally equipped with means for establishing a lower limit for the drying action; however, this can present problems under certain circumstances, for example, when the operating conditions are such that one cannot ensure the evaporation of a minimal quantity of moisture per unit of time. In such instances, even the aforementioned minimal or rock-bottom drying or heating action (quantity of transferred heat per unit of time) would lead to highly undesirable overdrying of tobacco particles.